As it is generally known, an internal-combustion engine comprises a closed lubrication circuit with an oil sump located in the lower part of the engine block, an oil pump for pumping the oil contained in this sump and passing it through an oil filter so as to free it of its impurities, and possibly through an oil radiator for cooling this filtered oil prior to feeding it back into the engine.
The main objective of lubricating oil is to facilitate the relative motion between two parts of the engine by minimizing or even preventing friction between these two parts. This oil therefore allows to create a viscous film that is interposed between these two parts so as to reduce or to prevent direct contact between the two surfaces of these parts.
This is notably the case for the piston of an engine that delimits, with the cylinder head and the cylinder, a combustion chamber wherein combustion of a fuel mixture occurs. This piston slides in the cylinder, with a rectilinear reciprocating motion, and a thin oil film must be continuously present between the outer surface of the piston and the wall of the cylinder. In the absence of this oil film, the motion of this piston undergoes a slowdown through friction in the cylinder, and the temperature of the piston and of the cylinder increases, which leads to a malfunction of this engine. This malfunction can lead to seizing of this piston, i.e. its sticking in the cylinder. Such seizure causes a sudden engine breakdown and requires a stage of reconditioning this engine, which is expensive.
This problem is all the more worrying since, for some types of diesel engines with combustion effluent post-treatment, the fuel injection strategies are such that part of the fuel injected is diluted in the oil film. In fact, during the engine running cycle, this fuel is fed into the combustion chamber in such a way that it is sprayed onto the oil film present on the major part of the axial area of the cylinder wall. This fuel mixes then by dilution with the oil of the film.
This oil containing diluted fuel then reaches the oil sump, generally through gravity under the effect of the scraper ring provided around the piston, from where it is pumped in order to be filtered and sent back towards parts to be lubricated, notably for creating the oil film between the piston and the cylinder. This film thus receives more fuel that is also diluted in the oil film. This oil with a still higher fuel dilution flows back into the oil sump where it is sucked by the pump for a new lubrication cycle.
Thus, after a certain number of lubrication cycles, the dilution ratio of the fuel in the oil is such that the oil film is impossible to obtain or it forms only locally between the piston and the cylinder, or it breaks upon motion of the piston. The two parts are therefore in direct contact, locally or totally, which slows down the motion of the piston through friction and increases its temperature, as well as that of the cylinder wall, eventually causing seizure of this piston.
The present invention aims to overcome the aforementioned drawbacks by means of a method allowing to remove the fuel present in the lubricating oil before it is fed back into the engine.